Fix missing cast warning (#471)

Source/astcenc_find_best_partitioning.cpp

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: Apache-2.0
 // ----------------------------------------------------------------------------
-// Copyright 2011-2023 Arm Limited
+// Copyright 2011-2024 Arm Limited
 //
 // Licensed under the Apache License, Version 2.0 (the "License"); you may not
 // use this file except in compliance with the License. You may obtain a copy
@@ -425,8 +425,8 @@ static unsigned int get_partition_ordering_by_mismatch_bits(
  }
 
  // Create a running sum from the histogram array
- // Cells store previous values only; i.e. exclude self after sum
- unsigned int sum = 0;
+ // Indices store previous values only; i.e. exclude self after sum
+ uint16_t sum = 0;
  for (unsigned int i = 0; i < texel_count; i++)
  {
  uint16_t cnt = mscount[i];